1. Field of the Invention
The present invention relates to an optical semiconductor device and, in particular, to an optical semiconductor device having a quantum well structure.
The technology of so-called band engineering regarding a quantum well structure made up of III-V compound semiconductor thin layers, super lattices, strained lattices or the like have been developed considerably. Optical devices such as lasers, optical sensing devices, optical modulating devices, and the like, in which band engineering is applied, have recently come into practical use.
2. Description of the Related Art
Regarding a structure in which there are two types of semiconductor layer, i.e., a barrier layer 42 and a well layer 44 shown in FIG. 1(a), an energy band diagram of this structure is shown in FIG. 1(b). That is, a quantum well 48 which corresponds to the well layer 44 is sandwiched between barrier layers 46 of valence and conduction bands corresponding to the barrier layer 42, and well-type potential continues. When the width L.sub.w of this well is as small as several 10 nm, the motion of the electrons and holes inside the well layer 44 in the z direction is limited, and respective energy levels are quantized. A well in such state is called a quantum well (QW), and a plurality of such wells when connected are called a multi-quantum well (MQW).
In the optical absorption spectrum of a bulk semiconductor material, generally, a sharp absorption peak is observed in the vicinity of the absorption edge at low temperatures. This absorption peak is due to an absorption caused by an exciton in which an electron and a hole are combined with each other by the Coulomb force. In a model of an exciton in which an electron is distributed spherically around a hole, its diameter is determined by the reduced effective mass of the exciton and the static dielectric constant of a semiconductor material, and is usually 20 to 40 nm.
When an exciton is produced in the quantum well 48, the bonding energy increases since the exciton is compressed in a direction perpendicular to the well layer 44, thus causing an increase in the oscillator strength of the exciton, i.e., the strength of optical absorption. A clear optical absorption peak is observed even at room temperature. An actually observed optical absorption spectrum is schematically shown in FIG. 2.
In FIG. 2, an absorption peak due to an exciton associated with an electron and a heavy hole is observed in the portion indicated by the letter A, and an absorption peak due to an exciton associated with an electron and a light hole is observed in the portion indicated by the letter B.
A quantum well having such optical characteristics is utilized in optical devices such as optical modulators, optical bistable devices, and optical switches.
A conventional optical modulator is a diode formed by an arrangement in which an active layer 50 formed from a MQW, in which the barrier layer 42 and the well layer 44 are laminated, is interposed between a p-type semiconductor 52 and an n-type semiconductor 54, as shown in FIG. 1(c). For the p-type semiconductor 52 and the n-type semiconductor 54, InP, etc. is used. The energy band diagram of this diode is the same as that shown in FIG. 1(b). When a reverse bias is applied to this diode, the energy band diagram varies as shown in FIG. 1(d). That is, the energy level of the quantized electrons and holes in the quantum well 48 sandwiched by barrier walls 46 are decreased. For this reason, the optical absorption spectrum is shown by the spectrum indicated by the broken line in FIG. 2. The absorption peak due to an exciton associated with an electron and a hole shifts several 10 meV toward the long wavelength side, i.e., a low energy side. This is called an electric-field on state.
Suppose that the light emission wavelength .lambda..sub.L1 of a semiconductor laser has been set to a position indicated by an arrow in FIG. 2, i.e., to the optical absorption peak A' of an exciton near the absorption edge in the electric-field on state. Optical modulation can be performed by causing this laser beam to enter the MQW active layer 50 of an optical modulator in which the electric field is turned on and off. That is, a laser beam is emitted from the MQW active layer 50 at a light intensity of I.sub.OFF, scarcely being absorbed in the off state. In the on state, however, the laser beam is absorbed in the quantum well 48 of the MQW active layer 50, and a light intensity I.sub.ON at which light is emitted to outside is small. The intensity ratio .eta. (=I.sub.ON /I.sub.OFF) of an emission laser beam which is emitted in correspondence to the on or off state of an electric field should preferably be smaller from the viewpoint of modulation characteristics. For that purpose, the strength of optical absorption due to an exciton should be as large as possible.
A conventional optical bistable device is one called SEED (self electro-optic effect device) in which the diode shown in FIG. 1(c) is connected to a power supply via an external load resistance. In the SEED, an incident beam is made to enter from a p-type semiconductor layer and an emission beam is caused to be emitted from an n-type semiconductor layer. Now, a reverse bias is applied to the SEED beforehand, and the wavelength .lambda..sub.L2 of the incident beam is set to the wavelength of an exciton absorption A in the electric-field off state. This incident beam is absorbed in the quantum well 48 of the MQW active layer 50, and a photoelectric current flows in the SEED at almost 100% efficiency. As a result, the reverse bias voltage applied to the diode is decreased, and the optical absorption spectrum shown in FIG. 2 nears the solid line, i.e., the electric-field off state. Therefore, an optical absorption coefficient with respect to the incident beam is increased, thus causing a photoelectric current to increase and causing the reverse bias voltage to further decrease. As the result of the occurrence of such a positive feedback, optical bistable characteristics can be observed between the incident beam and the emission beam. The width of this bistable area depends on the ratio of the absorption coefficient of the exciton absorption A indicated by the solid line in FIG. 2 to the absorption coefficient of the spectra indicated by the broken line corresponding to the wavelength .lambda..sub.L2 of the incident beam. The larger the ratio, the wider the bistable area.
FlG. 3 explains an example of an optical switch using a diode having an MQW 60 as in FIG. 1(c). An n-type InP layer 63 and a p-type InP island 62 are formed on an n-type InP substrate 64 via the MQW 60. Further, an X waveguide 61 of an n-type InP layer is disposed with the p-type InP island 62 in the middle. The black section of the upper section of the figure denotes an electrode. An incident beam I propagates the waveguide and travels straight as a transmission beam T in a case where an electric field is not applied to the MQW. Conversely, when an electric field is applied to the MQW, since the exciton absorption peak shifts toward the long wavelength side, the index of refraction varies. As a result, the incident beam travels another waveguide as a refraction beam R. The stronger absorption intensity by the exciton, the larger the variation in the index of refraction, and therefore switching characteristics are better.
As described above, the optical absorption strength of an exciton should preferably be larger in optical modulators, in optical bistable devices, and in optical switches. In the case of a III-V compound semiconductor, the optical absorption coefficient of an exciton is 1.times.10.sup.4 cm.sup.-1 at most at room temperature, and in the case of InGaAs, that is 8.times.10.sup.3 cm.sup.-1. To obtain excellent characteristics of an optical modulator, the value of the exciton absorption coefficient must be further increased 10% or more. A larger value of the absorption coefficient also has been desired to improve optical bistable device characteristics.